The activity and function of cortical circuits depend on the interplay between two interconnected sets of cells: excitatory principal neurons and inhibitory interneurons. Inhibitory interneurons play a critical role in preventing the circuit excitability that leads to epileptiform activity, and also are involved in a number of other processes as well, including regulation and synchronization of firing in principal neurons, release of neuromodulatory peptides, and somatodendritic inhibition of principal neurons. These functions are triggered by interneuronal activation, which occurs through excitatory synaptic transmission onto these cells. This excitatory transmission is primarily glutamatergic, and the excitatory postsynaptic current (EPSC) is mediated largely by ionotropic glutamate receptors of the AMPA receptor (AMPAR) and kainate receptor (KAR) subtypes. We have studied the AMPAR/KAR EPSCs onto hippocampal interneurons located in stratum radiatum and stratum lacunosum-moleculare (SR/SLM). We have found that the EPSC generated by extracellular stimulation has two kinetically distinct components: a rapid, large EPSC that is mediated by AMPARs and is generally similar to AMPAR EPSCs seen throughout the CNS; and a slow, small EPSC that is mediated by both KARs and AMPARs. This slow EPSC lasts for hundreds of milliseconds, which is completely unexpected based on the kinetics of heterologously expressed AMPARs/KARs in response to brief pulses of glutamate. The AMPAR component of the slow EPSC, but not the KAR component, can also be massively potentiated by inhibition of glutamate uptake. These results suggest that the kinetics of the slow EPSC must be determined by a prolonged glutamate transient, or by factors within the interneuron that alter the kinetics of native receptors compared to heterologously expressed receptors, and that the answer may differ depending on the receptor subtype involved and the efficacy of uptake mechanisms. In the proposed research, we will examine the mechanisms underlying the slow EPSC. We will use whole-cell voltage-clamp recording of SR/SLM interneurons in hippocampal slices to record the EPSC, and a variety of pharmacological and genetic manipulations to affect glutamate release, reception, and uptake. Our specific aims are: 1) to test whether the slow AMPAR EPSC is generated by glutamate spillover; 2) to determine whether the receptors underlying the slow EPSC have kinetics that are dictated by receptor subunit composition or interactions with scaffolding proteins; and 3) to define the role of glutamate uptake mechanisms in regulating the slow EPSC.